1. Field of the Invention
The present invention relates to an electrophotographic image forming device, such as a laser printer, a digital copier, and a facsimile. More particularly, the present invention relates to a process cartridge developing an electrostatic latent image formed on a photosensitive body into a developer image, and an image forming device having the same.
2. Description of the Related Art
Generally, an electrophotographic image forming device, such as a laser printer, a digital copier, and a facsimile, includes a process cartridge integrating a photosensitive body and a developing unit into a single module. The photosensitive body is exposed to a laser to form an electrostatic latent image thereon, and the developing unit supplies developer to the photosensitive body to form a developer image corresponding to the electrostatic latent image. Generally, the process cartridge is detachably mounted in an image forming device body to easily repair all parts of the cartridge.
FIG. 1 is a view of a conventional process cartridge 1 that is detachably mounted in the image forming device body.
The process cartridge 1 includes a photosensitive body 6 that is rotatably disposed in a photosensitive body casing 2.
A developing unit 4 is disposed adjacent to the photosensitive body 6.
The developing unit 4 includes a developing casing 8 formed integrally with or detachably from the photosensitive body casing 2. A developing roller 12 contacts the photosensitive body 6 with a space therebetween in the developing casing 8. A supply roller 13 supplies developer to the developing roller 12. A developer regulating blade (not shown) contacts the developing roller 12 to regulate the thickness of developer layer.
As shown in FIG. 2, a photosensitive body gear 6a formed on a photosensitive body shaft 6b is connected through an idle gear 10 and a retardation gear 11 with a developing roller gear 12a and a supply roller gear 13a in the developing casing 8. The photosensitive body gear 6a is also meshed with a driving gear 5 of a gear train connected with a driving motor (not shown) mounted in the image forming device body as the process cartridge 1 is mounted in the image forming device body.
Accordingly, as the process cartridge 1 is mounted in the image forming device body and the driving motor is driven, the photosensitive body gear 6a is rotated in a certain direction, such as clockwise, by the driving gear 5. As the photosensitive body gear 6a rotates clockwise, the photosensitive body 6, formed coaxially with the photosensitive body gear 6a, also rotates clockwise.
The rotating force of the photosensitive body gear 6a is transmitted through the idle gear 10 and the retardation gear 11 to the developing roller gear 12a and the supply roller gear 13a, and accordingly, each of the developing roller 12 and the supply roller 13 is rotated counterclockwise.
However, in the conventional process cartridge 1 with the above structure, the developing roller 12 and the supply roller 13 of the developing unit 4 are rotated with the photosensitive body 6 by the idle gear 10, the retardation gear 11, the developing roller gear 12a, and the supply roller gear 13a. Therefore, when the electrostatic latent image is not developed, for example, when the photosensitive body 6 idly rotates, the developing roller 12 and the supply roller 13 rotate together with the photosensitive body 6. The unnecessary rotating of the developing roller 12 and the supply roller 13 increases the developer stress because friction force is generated between the developing roller 12 and the supply roller 13, and between the developing roller 12 and the developer regulating blade. The increase of developer stress results in a decrease of development uniformity and image quality.
Additionally, the developing roller 12 and the supply roller 13 are connected with a plurality of gears, such as the idle gear 10, the retardation gear 11, the developing roller gear 12a, and the supply roller gear 13a. Therefore, when the velocity ratio between the photosensitive body 6 and the developing roller 12 is adjusted to control the developing capability, the velocity ratio cannot be easily adjusted because of the number of gear tooth and the adjustment of distances between shafts of gears.
To solve the above problem, a process cartridge (not shown) is introduced and used in which the idle gear 10 is omitted between the photosensitive body gear 6a and the retardation gear 11, and the driving force transmitted from a separate driving motor (30, referring to FIG. 3) is transmitted to a developing roller shaft 12b so that the developing roller 12 and the supply roller 13 of the developing unit 4 are driven separately from the photosensitive body 6.
As shown in FIG. 3, the process cartridge has the developing roller gear 12a formed in the developing casing 8, and therefore, a coupling device 10 is required for coupling a driving shaft 30a of the driving motor 30 and the developing roller shaft 12b formed in the developing casing 8 to transmit the driving force of the driving motor 30 to the developing unit 4.
The coupling device 10 includes a protrusion portion 17a, a recess 18a, and an elastic spring 21. The protrusion portion 17a is configured at a first coupling shaft 17 formed at a drum flange 16 of the developing roller shaft 12b to be coaxially formed with the developing roller gear 12a, and has a certain shape, such as a triangular shape. The recess 18a of a triangular shape is configured at a second coupling shaft 18 of a rotation member 15 to receive the protrusion portion 17a. The elastic spring 21 is formed at an image forming device body to elastically pressurize the second coupling shaft 18 to the first coupling shaft 17 so that the protrusion portion 17a can be coupled with the recess 18a. 
The rotation member 15 receives a driving force through a driven gear 34, which is meshed with a driving gear 20 of a driving shaft 30a of the driving motor 30, from the driving motor 30.
However, the coupling device 10 has a complicated operation, in which the protrusion portion 17a is fitted in the recess 18a as the cartridge is mounted, and requires a plurality of parts. Therefore, the manufacturing costs increase.
Additionally, when the developing roller shaft 12b engaged in the developing casing 8 is not stably coupled with the driving shaft 30a of the driving motor 30, the driving force may be inferiorly transmitted or not transmitted to the developing roller 12 and the supply roller 13 of the developing unit 4.
Accordingly, a need exists for an image forming device having an improved process cartridge in which developer stress is substantially prevented, such that image quality does not substantially deteriorate due to developer stress.